1. Field of the Invention
The present invention relates to a data processing apparatus, a printing apparatus, and a method of creating a mask. More particularly, the present invention relates to a dot data generation processing for dividing dot data with the use of a mask into respective dot data to be used for a plurality of times of scanning of a print head, and based on the divided dot data, performing printing with the use of a plurality of arrays of printing elements or a plurality of heads, in regard to printing elements for the same color.
2. Description of the Related Art
With the diffusion of information processing equipment such as personal computers in recent years, printing apparatuses as image forming terminals have also been rapidly developing and diffusing. Of those various printing apparatuses, an ink jet printing apparatus that executes ink ejection to perform printing on a print medium such as paper, cloth, plastic sheet and OHP sheet in particular has become mainstream in regard to personal use. And this is because such an ink jet printing apparatus has excellent advantages such as being of low-noise and non-impact type printing, high-density and high-speed printing operations, easy adaptable for color printing, and of low-cost.
Advances in ink jet printing technique have been facilitated image quality improvement, faster and more economical printing, thereby contributing to the diffusion of printing apparatuses into personal users. The diffusion of personal computers and digital cameras has also contributed to the diffusion of printing apparatuses. These digital cameras include the digital camera that functions alone, as well as the digital camera that is integrated into other device, for example a mobile phone. Due to such extensive diffusion, personal users have also been requiring more improvement of image quality. Particularly, in recent years, a print system in which photographs can be readily printed at home and the printed result has an image quality comparable to silver salt photographs have been required.
In ink jet printing apparatuses, granularity has so far been seen as a problem when compared to silver salt photographs. Various measures have been proposed in order to reduce such granularity. For example, known is an ink jet printing apparatus equipped with an ink system in which light cyan and light magenta whose color material concentration are lower are added to regular cyan, magenta, yellow and black. In such an ink jet printing apparatus, the granularity can be reduced by using ink such as light cyan and light magenta in a low image density region. Meanwhile, in a high image density region, a wider color reproduction range and smooth gradation can be realized by using regular cyan and magenta inks when printing. There is another method for reducing the granularity by designing smaller size of dots to be formed on a print medium. This can be generally realized by reducing the volume of an ink droplet to be ejected from an ejection opening of a print head. In this case, it is possible to print a high resolution image without reducing printing speed by reducing the volume of ink droplets as well as having more ejection opening at higher arrangement density.
On the other hand, high-image-quality and high-speed printing capabilities are also requirement of recent printing apparatuses. As a configuration for realizing such capabilities, a printing method in which a plurality of nozzle arrays are provided for ink of one color is described in Japanese Patent Laid-open No. 2005-262788. FIG. 1A shows a print head for the case where printing is performed with a single nozzle array provided for ink of one color. On the other hand, FIG. 1B shows a print head used for the case where printing is performed with two nozzle arrays provided for ink of one color. In addition, in these Figures, each nozzle array is shown as being composed of 16 nozzles, for illustrative simplicity. If an image, which should be printed with the a single nozzle array shown in FIG. 1A, is printed with the two nozzle arrays shown in FIG. 1B, the scanning speed of the nozzle arrays can be doubled and thereby the printing speed can be doubled even if a driving frequency for each of the two nozzle arrays is the same as in the case where printing is performed with the single nozzle array. Also, in the case where the two nozzle arrays shown in FIG. 1B are used, the number of times a print head for printing is used for a certain area, for example, a region for one scanning, is halved in comparison with the single array case shown in FIG. 1A, and thereby extends the lifetime of the print head.
A process for distributing dot data to a plurality of nozzle arrays such as two or more arrays is performed by, for example the configuration shown in FIG. 2. FIG. 2 shows the configuration from the step of dividing image (dot) data for so-called multi-pass printing to the step of performing printing by driving a print head based on the divided dot data. In FIG. 2, the image (dot) data input in step 201 is subject to mask processing in step 202 to generate the dot data for each of a plurality of times of scanning. Then, in step 203, the divided dot data for each scanning is allocated to the nozzles of each nozzle array. The allocation of the dot data to the nozzle arrays is performed according to predetermined patterns. The patterns are referred to as “block patterns” or “driving patterns” in the present specification.
FIG. 3 is a schematic diagram illustrating particularly the block patterns (driving patterns) for allocating ejection data to two nozzle arrays.
The example illustrated in the diagram shows that each of two nozzle arrays A and B that eject ink of the same color comprises 16 nozzles arranged vertically in the diagram at intervals corresponding to 1200 dpi. Also, the image to be printed is a solid image (an image in which all the areas (area: a unit region to be printed with an ink dot) are printed with ink dots) with a resolution of 1200 dpi. Furthermore, the driving frequency for each nozzle of the nozzle arrays A and B is intended for one ejection at area intervals corresponding to 600 dpi. In addition, in the case of multi-pass printing, the image to be allocated to the two nozzle arrays is a divided image correspondingly to each of a plurality of times of scanning. Regarding the above solid image, in each scanning, ink dots are printed on areas corresponding to a division ratio based on the number of times of scanning; however, in the following description with reference to FIG. 3, the case where the solid image is printed in one scanning is described as an example for descriptive simplicity.
In FIG. 3, dot data is allocated to the nozzle arrays A and B based on the block patterns A and B, respectively. Specifically, when the column (a vertical area array in the diagram) 0 is printed, nozzles with nozzle numbers {1, 2}, {5, 6, 7 }, {9, 10} and {14} are used in the nozzle array A, and those with nozzle numbers {3, 4}, {8}, {11, 12, 13} and {15, 16} are used in the nozzle array B. When the column 1 is printed, nozzles exclusive of (complement to) those used for the column 0 are used for printing in both of nozzle arrays A and B. In addition, if n nozzle arrays, where n is 3 or more, are used, it should be appreciated that patterns are complemented by n columns. Also, the example illustrated in FIG. 3 shows that a plurality of nozzle arrays for ink of the same color are configured as one head. However, it should be appreciated that the above description is also applicable even if each nozzle array is configured as one head and a plurality of print heads are used for ink of the same color.
As described, even if each nozzle is driven with the driving frequency corresponding to one ejection at 600 dpi intervals, driving equivalent to that with the driving frequency corresponding to one ejection at 1200 dpi intervals can be achieved, so that printing of a high-resolution image can be achieved without reducing printing speed.
Meanwhile, if a printing speed is increased in general, for example, if a printing configuration in which a plurality of nozzle arrays such as the above 2 nozzle arrays are provided and a printing speed is increased by several times without the change in driving frequency is employed, a problem of beading in a printed image may arise.
That is, the increase in speed causes the increase in an amount of ink applied to a unit area of a printing medium per unit time. In such a case, although the printing medium may eventually absorb a total amount of the ink applied depending on a type of the printing medium, the absorption may not be able to follow the applying speed, and therefore ink droplets, which have not been absorbed at the surface of the printing medium, may come into contact with each other during printing. Then, a relatively increased size of ink droplet caused by combination due to the contact may become highly visible on a resulting image to thereby reduce image quality.
For example, it is considered that a blue image represented by cyan ink and magenta ink is printed onto a printing medium by multi-pass printing of two-pass. A serial type printing apparatus performs scanning with a print head in which nozzle arrays, for example, for 4 colors of cyan, magenta, yellow and black, are arranged for printing. During the scanning, the inks are ejected onto the same region of the printing medium from the respective nozzle arrays. In the case of printing the blue image, the cyan ink and the magenta ink based on dot data obtained by thinning image data for cyan and magenta into ½ respectively are applied onto the printing medium with a relatively short time difference in the same scanning. At this time, if the cyan ink and the magenta ink are applied onto the same area or adjacent areas, both are attracted each other by their surface tensions and therefore a large lump (hereinafter also referred to as a grain) of ink having the size of the two ink droplets (or more) is formed. Once such a grain is formed, ink applied to the position adjacent to the grain is likely to be attracted by the grain. That is, the first generated grain serves as a nucleus and gradually grows, and eventually a large grain is formed. As a result, such a grain itself or the presence of such grains that are irregularly distributed causes an adverse effect on an image, which is called “beading”.
Such a grain is not generated only by the surface tensions of ink droplets. For example, if inter-reactive printing liquids are applied in the same scanning, the contacted liquids are combined by a strong chemical reaction, which may cause the formation of a grain nucleus.
Various printing methods and mask patterns for preventing the beading have conventionally been proposed. Japanese Patent Laid-open No. 2002-144552 describes the use of a mask that is configured to improve the dispersibility in an arrangement of dots that is printed with one scanning of a nozzle array for ink of one color. According to this, the grain formation by the contact of ink droplets of the same color printed in one scanning can be decreased.
In Japanese Patent Laid-open No. 2002-144552, the dispersibility of dots in one plane printed by a nozzle array for ink of one color in one scanning is considered; however, dispersibility in relation to dots in another plane is not considered. For this reason, dispersibility in relation to, for example, dots in another color plane printed in the same scanning or dots in a plane printed in another scanning, is poor, which may cause grains.
The inventors of the present invention have found that such a problem of the dot dispersibility between different planes is applicable to that between the above-described dot arrangement by each block pattern when printed with a plurality of nozzle arrays for the same color and the dot arrangement by a mask.
FIGS. 4(a) to (e) are diagrams for explaining this problem. A pattern (a) shows image (dot) data 401 for one color to be printed. The dot data 401 is data for one scanning out of data for a plurality of scanning into which a mask divides.
Patterns (b) and (c) respectively illustrate block patterns 402 and 403 for the case where printing is performed with two nozzle arrays A and B as shown in FIG. 3. When the nozzle arrays A and B are driven based on the block patterns 402 and 403, the dot data 401 is printed as dot arrangement patterns 404 and 405 shown in patterns (d) and (e) respectively.
It is obvious from the patterns (d) and (e) that the dot pattern generated by the mask and each of the block patterns interfere with each other and therefore the nozzles of one of the two arrays are more used. Such uneven use of the nozzles is likely to result in poor dispersibility of printed dots to thereby cause beading due to the above-described grains. Thus, if the interferences in dot positions between the mask and a plurality of planes with a plurality of block patterns are present, the dispersibility in a dot arrangement of an image to be eventually printed may become poor, causing the problem of beading.